A programmable logic device (PDL) typified by a field programmable gate array (FPGA) is a semiconductor integrated circuit which is reconfigurable even after a chip is manufactured. An island-style of PLD comprises a plurality of identical computational units arranged two-dimensionally. The computational units comprise programmable wiring resources and programmable logic resources.
The programmable wiring resources each include a programmable crossbar which comprises a group of lines, another group of lines perpendicular to the above group of lines, switches for switching the mutual connection relationship between the groups of lines, and memories for recording a mutual connection relationship. The programmable crossbar is sometimes referred to as a programmable interconnect or a programmable switch matrix.
It is known in technique that a programmable crossbar is made to comprise a group of lines, another group of lines perpendicular to the former group of lines, and a plurality of resistance change elements provided at intersections of those groups of lines. Each of the resistance change elements is a two-terminal nonvolatile resistance change element, and its state can be switched between a low resistance state and a high resistance state by applying a predetermined voltage between terminals of the nonvolatile resistance change element.
To be more specific, a programmable crossbar in a computational unit is connected to a programmable logic resource in the computational unit, and also to another computational unit. In addition, the programmable crossbar is also connected to a program control circuit. The program control circuit comprises a program voltage control circuit and program lines. The program control circuit is shared among the plurality of computational units. It is therefore necessary that the program lines are connected to programmable crossbars in the computational units by selectors. During a program operation, the selectors connect the program control circuit and a selected programmable crossbar, and during a PLD operation, the selectors disconnect the programmable crossbars from the program control circuit.
Also, it is known in technique that a selector is made to comprise a three-terminal metal-oxide-semiconductor field-effect-transistor (MOSFET). The MOSFET is sometimes referred to as a programming transistor. The programming transistor needs to have a sufficient withstand voltage or a sufficient current driving performance for transmitting a program voltage pulse from a two-terminal nonvolatile resistance change element. Thus, there is a case where its footprint is larger.
It is required to provide a technique for increasing the logic density of a PLD. The logic density can be increased by applying programmable crossbars employing two-terminal resistance change elements. Furthermore, in order to increase the logic density, it is required to provide programmable crossbars including selectors which can be provided in a smaller footprint, and a program operation method and a PLD operation method for the programmable crossbars.